In the control of various conditions, such as temperature, various control systems have been provided. For example, temperature controlling valves driven by a motor controlled by a thermostat have been employed to control the temperature of a space. These systems, however, have been on-off systems in the sense that the motors either drove the valves fully opened or fully closed. In such systems, the condition or temperature responsive device operated between an off position and an on position. The distance between the off and on positions is termed the "differential" of the control system. Because these on-off systems operated within a differential, the actual temperature of the controlled environment tended to oscillate around the control or set point. To eliminate this cycling, the prior art developed the proportional control system.
To make the prior art motor driven or actuator driven on-off systems proportional, a rebalancing resistor connected into a bridge circuit with the condition responsive device and operated by the motor or actuating means was included. Thus, when the condition responsive means sensed a change in condition away from a control point, the motor or actuator would move a control element such as a valve a distance until the balancing resistor balanced the condition responsive means. This distance is then proportional to the condition responsive means, i.e. the deviation of the actual sensed condition from the set point. These prior art proportional systems, however, tend to be complicated and expensive.